Fixed nitrogen loss from the eastern tropical North Pacific and Arabian Sea oxygen deficient zones determined from measurements of N2:Ar

Abstract

Previous work estimating the N2excess above background due to denitrification has suggested that nitrate deficit‐type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N2excess approach has the advantage over nitrate deficit‐type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. Measurements of N2:Ar from two stations within the eastern tropical North Pacific and from one station within the Arabian Sea oxygen deficient zones were used to determine the N2 excess due to denitrification. In both of the regions, the N2 excess was comparable in shape and magnitude to the concurrent fixed nitrogen deficit. In the eastern tropical North Pacific oxygen deficient zone, the N2 excess was near zero at the surface and rose to maxima of 13.7 ± 1.8 and 10.8 ± 1.9 μM N, compared to maximum fixed N deficits of 13.5 ± 1.5 and 12.3 ± 1.5 μM N, respectively. In the Arabian Sea oxygen deficient zone, the maximum N2 excess was 11.1 ± 1.5 μM N, compared to a maximum deficit of 12.5 ± 1.0 μM N. These results suggests that previous estimates of fixed N loss based on fixed N deficit calculations in these regions are likely reasonable, given the same considerations of volume and residence time of the water of the oxygen deficient zone.